Consider only handling post-fulu gossip

> we should consider if we only wanna handle post-fulu here and drop all other cases, could simplify this code a bit

_Originally posted by @nflaig in [#9762](https://github.com/ChainSafe/lodestar/pull/9762/changes#r3711081642)_

Start with the changes in pull request #9762 and locate the gossip-handling code discussed in the quoted comment. Determine which cases are post-Fulu and confirm the intended behavior with existing tests; done means non-post-Fulu cases are no longer handled without changing post-Fulu behavior.

Written by the indexing model from the issue text.